The invention relates to an abrasive tool with a supporting body and, connected thereto, at least one abrasive element with sintered metal bonded abrasive grain. The invention further relates to a method for manufacturing and use of such a tool. In particular, the invention relates to the manufacture and use of grinding, abrasive cutting, sawing or wood drilling tools made of diamond or respectively cubic boron nitride-containing sintered metallic bonded cutting segments, and supporting bodies of steel, to which the cutting segments are joined by brazing, welding or direct sintering on.
Such tools are used for shaping, cutting and drilling metal, glass, natural stone, artificial stone, concrete, ceramics, and plastics reinforced or not with fibres or fillers. These are abrasive processes for wet and dry use. The actual cutting material used is preferably a high performance abrasive medium such as cubic boron nitride or diamond, with grain sizes of 150 to 900 xcexcm.
A task of the same importance as that of the cutting material is satisfied in abrasive tools according to the invention by the sintered metal bond. The following properties and tasks for an economic sintered metal bond for abrasive tools are known from the prior art:
There must be sufficient retention of the abrasive grain to prevent premature loss of abrasive grain. If the bond matrix is too soft, the abrasive grains can be loosened in their bond environment by the cutting forces on the cutting edges of the grains, which leads to premature grain loss with an uneconomical operating result. When too many cutting grains fall out prematurely, this also results in the operating conditions being made more difficult, characterised be high frictional losses on the contact surface between the body of the bond and the material to be worked on. Such operating conditions are manifested by high poster consumption, reduced drilling, grinding or cutting progress, and are often associated with increased noise and emission of sparks.
A further manifestation of excessively soft bond characteristics with respect to the diamond or cubic boron nitride high performance abrasive materials, is the risk of the cutting grains being impressed or pushed further into the bond. This risk occurs particularly when working without liquid cooling. FIG. 1 shows schematically the displacement of abrasive grain in a bond that has become pasty due to the influence of excessive heat. The effect is, as described hereinabove, frictional loss, high power consumption, spark emission, poor cutting performance and loud noise, as the cutting edges of the abrasive grains disappear beneath the surface of the bond.
In addition to providing retention for the grain, the wear behaviour of the bond must be optimised with respect to the workpiece material and its chippings, with respect to the work settings, and with respect to the cooling agent, namely air or cooling liquid. If the bond matrix wears excessively, the abrasive grains are prematurely worn out by the abraded chippings. Uneconomical working is again the result, as the grains are lifted out too quickly. When, on the other hand, the bond is made too wear-resistant, the abrasive grain is retained for too long, and this can lead to blunting because of the cutting edges becoming rounded, and thereby lead to a loss of the cutting properties.
In the manufacture, in particular in the mass-production of grinding, abrasive cutting and drilling tools containing abrasive grain for the construction and stone industries, the pressurised sinter compression method is used as a rule. The principle of short circuit current heating or respectively inductive heating is used to generate the heat required for the process.
For example, machines commercially available from the Dr Fritsch (DSP25AT, SPM 75), Sintris (18STV, 19ST3T) or Arga (CAR1001) companies can be used for this. With this, multi-part sintering moulds according to FIG. 2 are used, in which during sintering a thermal gradient generally occurs, as in the center of the segment to be sintered a temperature occurs sometimes up to 40xc2x0 C. higher than in the outer areas of the segment. In pressurised sintering, which is normal in practice, when there is a liquid phase, this is more strongly expressed in the center region than in the edge regions, which leads to undesired non-homogeneity such as variation in mass texture and hardness, and can lead to blowing out.
The base metal mainly used for many years is cobalt. This metal has limited availability in terms of reserves. The price of cobalt is the subject of speculative transactions, in the same way as the price of silver or gold. Continuously increasing pressure on prices in the metal bonded high performance abrasives sector is forcing manufacturers of those materials to research alternatives. The replacement of cobalt with a single replacement material has proved technically impossible. These days, iron seems the most likely basic raw material as the price of iron is low and not the subject of speculative transactions.
The soft iron can be made slightly harder with copper. The maximum solubility of copper in xcex1 iron is 1.4 percent by weight, at 850xc2x0 C. Tin makes iron harder, but also more brittle and can therefore only be used in small quantities in alloys (F. Rapatz: Die Edelstahle, 1962). In iron-copper alloys, carbon has a hardening effect by forming carbides with iron and by its effect on the xcex3-xcex1 transformation, but also causes brittleness and is difficult to weld. For these reasons, the alloy according to the invention is advantageously not alloyed with any carbon.
The addition of tungsten carbide increases wear resistance in cobalt bonds. With iron bonds, improvement in near resistance is also possible, although because of the low degree of intrinsic hardness of the iron bond, only to a limited extent.
The object of the present invention is to provide the iron bond system with, in addition to copper (tin), further alloying partners which, at normal manufacturing temperatures for tools containing super abrasive agents, of between 800 and 1000xc2x0 C., satisfy as many as possible of the following requirements, namely
increasing the intrinsic hardness of the iron bond in order to prevent displacement or impression of the grains of cutting material when there are difficult operating conditions in their bond environment as represented in FIG. 1,
not causing brittleness, in order to make use, without breakage or respectively fissuring possible in manufacturing and in application,
transforming the liquid phase caused by tin as quickly as possible into a solid alloy phase,
retaining the high performance abrasive medium for as long as possible in the bond,
not in any way chemically, thermally or mechanically damaging the high performance abrasive medium,
matching as well as possible the bond wear to the wear of the high performance abrasive medium,
particles being obtainable in sufficient quantity and sizes,
available at an acceptable raw material price,
as environmentally friendly as possible.
The object of the invention is above all an abrasive tool according to claim 1.
The different metal carbides, metal borides and metal silicides react to a small extent with the bond metals iron, copper and tin. The metal carbides of chromium, molybdenum and titanium react with iron and copper on the contact surfaces and thereby cause hardening of the bond metal by forming an intermetallic phase, and the particularly flood integration of these hardening materials. Chromium boride reacts to a small extent with iron, forming an intermetallic phase. These hardening materials are well bonded with the matrix, and increase wear resistance. The silicides of chromium and molybdenum react with iron and form different iron silicides, which are hard but brittle. The content of these hardening materials in iron bonds has therefore to be very carefully adjusted.
By coordinating the hardening material with the iron-copper-tin matrix, all the properties set out hereinabove can be satisfied by the metal bond according to the invention. It as shown that in order to satisfy all the requirements set out hereinabove, at least two metal carbides, metal borides, metal silicides or combinations thereof must be alloyed with the soft bond matrix. The more complex the task to be solved, the more hardening materials have to be used. Wear resistance can be further increased by the addition of tungsten carbide.
A feature of the alloy according to the invention is the obtaining of hardness values of up to approximately 120 degrees hardness according to Rockwell B (HRB) without any great loss of ductility. A bond according to the invention with approximately 10% coarse grain tungsten carbide and a hardness of 120 HRB achieves impact resistance of approximately 0.03 J/mm2. A standard cobalt based, bond of the same hardness achieves 0.02 J/mm2. An iron bond according to the prior art (with considerable addition of bronze, nickel and tungsten carbides) achieves only 0.01 J/mm2 and is no longer producible with sufficient reliability.
The hardness of the copper covered iron powder is approximately 85 HRB after sintering. Tin increases the hardness of the basis of the bond to approximately 95 HRB. The hardness can be increased to approximately 105 HRB with chromium carbide. By addition of further metal borides and/or metal carbides, the hardness set out hereinabove, of 120 HRB, is obtained.
Each bond component makes possible the improvement of a tool characteristic. Metal borides in combination with metal carbides increase the hardness of iron bonds and reduce the bond wear in use. Using tin, the sintering temperature can be reduced to temperatures at which abrasive tools can be manufactured without damaging the abrasive grain. Some metal carbides regulate the amount of fluid phase and by their addition, increase the process reliability. Hardening effects of iron based materials can be obtained below 850xc2x0 C. by addition of metal silicides.